PE Sample Question - Sizing a Branch Conductor

[jumper]

You don't have to round up in the rare case where staying put help you.

Agreed. The rule is permissive not mandatory.

(B) Fractions of an Ampere. Calculations shall be permitted
to be rounded to the nearest whole ampere, with decimal
fractions smaller than 0.5 dropped.

 

[jumper]

So, instead of a consistent treatment of significant digits, it allows you to round 8.1 down to 8, but not 81 down to 80.
Not uncommon outside scientific and engineering contexts.

I dunno, it says what it says. Go pick on someone else.

 

[rlqdot]

Question states "what size 90°C THHN conductors".

We all preach not to read too much into a question or assume.

I agree, the question asks about the CONDUCTOR - for all we know, there is a #8 CU THHN terminated to the OCPD and a 90 d C wire nut (or some other splicing device) connecting the #8 and #10 THHN.

When my office designs very long runs of conductors, we sometimes put splice boxes with high temperature connectors downstream of the OCPD so that we can take advantage of the 90 degree temp of the conductor and NOT size it by the termination temp.

 

[Carultch]

I agree, the question asks about the CONDUCTOR - for all we know, there is a #8 CU THHN terminated to the OCPD and a 90 d C wire nut (or some other splicing device) connecting the #8 and #10 THHN.

When my office designs very long runs of conductors, we sometimes put splice boxes with high temperature connectors downstream of the OCPD so that we can take advantage of the 90 degree temp of the conductor and NOT size it by the termination temp.

If you do something such as that, are you required to have the splice take place in an enclosure external to the enclosure that has 75C rated equipment? I don't see where the code says that it cannot be in the same enclosure as the panelboard with 75C breakers. All I've seen is the example schematics that people draw as an interpretation of this solution.

 

[Saturn_Europa]

Yes and I don't see how C answers the question. The question wants to know what size wire is required for this install. A #10 is not sufficient unless there are 90C termination. I am confused by what you 2 are thinking. The answer states #10 at 38 amps--This answer is worthless after all the calcs. The wire is noncompliant at 38 amps

This is my thought also. 240.4 states that overcurrent protection for 10# must not exceed 30A.

What is the work around? 430.28 430.52? But a distance was not given.

What am I missing?

 

[electrofelon]

They also did not add 25% to the 2.08 amps for being continuous. Granted it wouldnt change the answer, but the walk-through should be correct.

 

[Carultch]

This is my thought also. 240.4 states that overcurrent protection for 10# must not exceed 30A.

What is the work around? 430.28 430.52? But a distance was not given.

What am I missing?

Are you saying that even if you use a 90C rated splice to transition from a #8 whip on both ends for 75C rated equipment terminations, to #10 wire for the majority of the length, that you cannot have this circuit on any larger than a 30A OCPD?

Because that is the work-around we are discussing. However, given the maximum OCPD limits of #14/#12/#10, it sounds like that with few exceptions, this work-a-round only works for #8 and larger.

 

[Dennis Alwon]

Why is the 37.08A rounded to 38? because the calculated amps are >37, you go to 38? Where is that in the code? Why am I having a flashback to math class and the importance of significant digits?

I believe you may be think of the rules in notes to the tables in Chapter 9 but that is .8 not .08.

When calculating the maximum number of conductors or
cables permitted in a conduit or tubing, all of the same
size (total cross-sectional area including insulation), the
next higher whole number shall be used to determine the
maximum number of conductors permitted when the calculation
results in a decimal greater than or equal to 0.8.
When calculating the size for conduit or tubing permitted
for a single conductor, one conductor shall be permitted
when the calculation results in a decimal greater than or
equal to 0.8.